This invention relates to methods and compositions for electroless deposition of metallic silver on miscellaneous substrates. Electroless silver deposition, or as it is also known in the art as silver spray or aerosol deposition, is an old art finding utility in diverse products ranging from mirrors to decorative plastics and phonograph record masters. The state of the art and its evolution is well documented in many publications and issued patents. The following are just a few describing the subject and they are included herein by reference.
1. S. Wein, "Metallizing Non-Conductors," Finishing Publication, Inc., (1945). PA0 2. W. Goldie, "Metallic Coating of Plastics," Electrochemical Publication Limited, (1968), Vol. 1, and in particular Chapter II. PA0 3. d. j. levy et al, Plating, November (1965). D. J. Levy et al, Sampe Jr., April/May (1969). D. J. Levy et al, J. Electochem. Soc., 121, 484 (1974). PA0 4. U.S. Pat. Nos. 2,367,903; 2,439,654; 2,664,363; 2,822,289; 2,214,476; 2,602,757; 2,614,943; 3,963,842; 3,776,740 and 3,983,266.
In general, electroless silver (or spray) is deposited upon a properly sensitized substrate (or a proper catalytic surface) utilizing dual spray guns wherein the individual streams are atomized and react to precipitate by a chemical reduction metallic silver. Generally speaking, in the aerosol spray technique, there are at least two main streams encompassing two solutions; the first comprises a soluble silver salt along with a suitable complexing agent and the second is a reducing agent along with a pH adjustor. Typical compositions in the prior art have utilized silver nitrate as the source for the silver ions and ammonia as the complexing agent. Reducing agents commonly reported are, but are not limited to, hydrazine and its derivatives, formaldehyde, sugars, etc. Inherent with such process of spray, there is always a certain quantity of silver which precipitates in a sludge rather than the desired coating thereby reducing the efficiency of the deposition process. Thus, there has been a continuous search for means by which the efficiency of the deposition process can be improved through reducing the quantity of sludge formation. Examination and evaluation of commercially available solutions or solutions prepared from the above references still reveals the presence of comparatively high quantities of the undesired silver sludge formation. It is thus recognized that there is still a need for improved compositions which would provide a more efficient process for the silver spray deposition process or for the increased plating rate of electroless silver deposition.